docs: def get_instrument_variables(ds):
      '''
      Returns a list of instrument variables
  
      :param netCDF4.Dataset ds: An open netCDF4 Dataset
      '''
      candidates = []
      for variable in ds.variables:
          instrument = getattr(ds.variables[variable], 'instrument', '')
          if instrument and instrument in ds.variables:
              if instrument not in candidates:
                  candidates.append(instrument)
  
      instrument = getattr(ds, 'instrument', '')
      if instrument and instrument in ds.variables:
          if instrument not in candidates:
              candidates.append(instrument)
      return candidates